Bipolar diaphragm electrolytic cell having internal anolyte level equalizing means

ABSTRACT

Disclosed is a bipolar electrolyzer containing a plurality of individual diaphragm cells in bipolar electrical and mechanical configuration. Each pair of bipolar cells are separated by a backplate having a cathodic surface facing the first of the pair of cells and an anodic surface facing the second of the pair of cells. In the first cell of the pair of cells, an electrolyte permeable cathode, spaced from, and mechanically and electrically connected to the cathodic surface of the backplate, defines a catholyte volume between the cathode and cathodic surface of the backplate. Each pair of bipolar electrolytic cells has an equalizer which passes through the backplate between the pair of cells, and through the catholyte volume and cathode of the first of the pair of cells, providing hydraulic communication between the anolyte chambers of the first and second cells of the pairs. Also disclosed is a method of operating a bipolar electrolyzer having an equalizer passing through the catholyte volume of a cell and providing hydraulic communication between a pair of cells. In the disclosed method, the anolyte is transferred from an anolyte chamber of one cell through an equalizer passing through the backplate and catholyte volume of a cell to the anolyte chamber of an adjacent cell.

United States Patent [191 Raetzsch, Jr. et al,

[ Dec.3,1974

[ BIPOLAR DIAPHRAGM ELECTROLYTIC CELL HAVING INTERNAL ANOLYTE LEVEL EQUALIZING MEANS [75] Inventors: Carl W. Raetzsch, Jr.; Hugh Cunningham, both of Corpus Christi, Tex.

[73] ,Assignee: PPG Industries, Inc.,, Pittsburgh, Pa.

[22] Filed: July 20, 1973 [21] Appl. No.: 381,122

[52] U.S. Cl 204/256, 204/98, 204/255 [51] Int. Cl B0lk 3/10 [58] Field of Search 204/98, 128, 255, 256, 204/257 [56] References Cited UNITED STATES PATENTS 2,858,263 10/1958 Lucas et al. 204/256 FOREIGN PATENTS OR APPLICATIONS 264,361 6/1970 U.S.S.R .1 204/256 Primary Examiner-John H. Mack Assistant Examiner-W. I. Solomon Attorney, Agent, or FirmRichard M. Goldman 5 7 ABSTRACT Disclosed is a bipolar electrolyzer containing a plurality of individual diaphragm cells in bipolar electrical and mechanical configuration. Each pair of bipolar cells are separated by a backplate having a cathodic surface facing the first of the pair of cells and an anodic surface facing the second of the pair of cells. In the first cell of the pair of cells, an electrolyte permeable cathode, spaced from, and mechanically and electrically connected to the cathodic surface of the backplate, defines a catholyte volume between the cathode and cathodic surface of the backplate. Each pair of bipolar electrolytic cells has an equalizer which passes through the backplate between the pair of cells, and through the catholyte volume and cathode of the first of the pair of cells providing hydraulic communication between the anolyte chambers of the first and second cells of the pairs. Also disclosed is a method of operating a bipolar electrolyzer having an equalizer 3 Claims, 5 Drawing Figures PATENILDBEE aim 3.852.179

' sum 2 or a BIPOLAR DIAPHRAGM ELECTROLYTIC CELL HAVING INTERNAL ANOLYTE LEVEL EQUALIZING MEANS BACKGROUND OF THE INVENTION Bipolar diaphragm cell electrolyzers offer significant economies of construction and operation, especially at. high current densities, e.g., above about 125 Amperes per square foot, over monopolar diaphragm cells. Bipolar diaphragm cell electrolyzers are characterized by a backplate, which is also known as a bipolar unit or a bipolar electrode. The backplate serves as a common structural member supporting the cathodes of one cell in a bipolar electrolyzer and the anodes of the next adjacent cell in the bipolar electrolyzer. The backplate further serves as a means of conducting electrical current from the cathode of one cell in the electrolyzer to the anodes of the-next adjacent cell in the electrolyzer. The backplate is electrolyte-impermeable so as to prevent the mixing of catholyte of one cell and the anolyte of the next adjacent cell of the series.

Each cell has anodes and cathodes. The cathodes are electrolyte-permeable, and covered with a diaphragm. The diaphragm divides the cell into a catholyte chamber and an anolyte chamber.

In the operation of the bipolar electrolyzer, brine is fed into each of the separate cells and an electrical potential is imposed across the electrolyzer. The electrical potential causes current to flow from a power supply to an anodic end unit and from the anodic end unit through the individual cells in the electrolyzer, in series, to a cathodic end unit and then back to the power supply or to an adjacent bipolar electrolyzer in a cell plant. Chlorine is recovered from individual anolyte chambers, and hydrogen gas and cell liquor are recovered from individual catholyte chambers. In the case of a diaphragm cell, the cell liquor contains approximately 120 to about 225 grams per liter of sodium chloride and about 110 to about 150 grams per liter of sodium hydroxide. In the case of an electrolytic cell where a permionic membrane is used rather than a diaphragm, the cell liquor may contain up to 300 or more grams per liter of sodium hydroxide and substantially lesser amounts, e.g., less than about 80 grams per liter, of sodium chloride and most frequently less than 10 grams per liter.

The brine feed to the individual electrolytic cells of the electrolyzer is saturated brine containing from about 300 to about 325 grams per liter of sodium chloride. The brine is fed to the cell at an elevated temperature to maintain the sodium chloride in solution. Most frequently the brine is fed to the cell at a temperature from about l60 to about 200F.

It has been found that if the temperature of the brine in the brine feed to the cells should drop or if the salt content should increase, some deposition of sodium chloride crystals in the brine feed lines will occur. Most commonly this will occur in the brine feed orifices and at bends, joints, and discontinuities in the brine feed line. When a blockage occurs in the brine feed, the flow of brine to an individual cell in the electrolyzer is interrupted causing the anolyte level to drop. This may result in anomalies in the operation of an individual cell, e.g., the cathodes become exposed to chlorine gas, hydrogen enters the anolyte compartment through the diaphragrn, anolyte boils, and arcing occurs across the electrodes.

Should any of these anomalies occur, it is likely that catastrophic failure of the electrolyzer could follow. Accordingly, it has been found necessary to provide equalizer means in a bipolar electrolyzer. Equalizer means serve to maintain a uniform head of anolyte in the individual cells. The equalizing function is accomplished by providing hydraulic communication between individual cells in an electrolyzer.

' In bipolar electrolytic cells of the prior art, such as disclosed in U.S. Pat. No. 3,337,443 to Carl W. Raetzsch et al., and U.S. Pat. No. 2,282,058 to R. M. Hunter et al., maintenance of substantially equal anolyte heads in each of the individual cells was provided by seepage around the backplate between individual cells or by openings in the backplate, below the level of the cathodes. In still other bipolar diaphragm cells, for example, U.S. Pat. No. 3,236,760 to G. Messner for a bipolar hydrochloric acid cell, equalizing is provided in combination with the anolyte feed means, i.e., anolyte is fed to individual cells through a manifold or header below the level of electrolyte in the anolyte chamber, the feed manifold or header also serving as the equalizer. Such an arrangement is satisfactory in an electrolytic cell where the electrolyte feed is unsaturated and at a temperature and concentration that is far from conditions of potential saturation and crystallization. However, the combination of a single electrolyte feed and anolyte equalization means is not feasible in a chlorine cell where the feed is saturated brine approaching a temperature and concentration where sodium chloride may crystallize out, plugging the brine feed means.

Commonly assigned copending U.S. Patent Application Ser. No. 171,231, filed Aug. 12, 1971, now U.S. Pat. No. 3,755,l08, shows an external equalizing system. Such an external equalizing system while satisfactory from an operational point of view does offer increased costs and risk of failure as a result of its external location.

SUMMARY It has been found now that a superior equalizer for bipolar chlorine diaphragm cells may be provided by an internal equalizer passing through the backplate, through the catholyte, and through the cathode, providing hydraulic communication between adjacent anolyte chambers. Such an equalizer carries anolyte from one cell through the backplate, the catholyte chamber, and the cathode of a cell to an adjacent cell providing anolyte equalization.

DETAILED DESCRIPTION OF THE INVENTION Our invention may be understood with particular reference to the appended Figures. In the Figures:

FIG. 1 shows an exploded isometric view of a bipolar electrolytic diaphragm cell of this invention.

FIG. 2 shows a partial cutaway perspective view of a backplate containing an equalizer means of this invention.

FIG. 3 is a cutaway plan view of an electrolytic cell backplate showing one exemplification of an equalizing means of this invention.

FIG. 4 is a cutaway plan view of an electrolytic cell backplate of another exemplification of the equalizing means of this invention. 7

P16. 5 is a cutaway plan view of an electrolytic cell backplate of another exemplification of an equalizing means of this invention.

A typical bipolar diaphragm cell is shown in partial exploded view in FIG. 1. The electrolytic cell 1 has a cell body 11 containing a number of individual bipolar units 21. Each bipolar unit 21 has anodes 31 extending from an anodic surface 25 of the backplate: 23 and cathodes 41 extending from a cathodic surface 27 of the backplate. A cathode backscreen 43, spaced from and mechanically and electrically connected to the cathodic surface 27 of the backplate 23, defines a catholyte volume between the cathode backscreen 43 and the cathodic surface 27 of the backplate 23.

The anodes 31 of the one bipolar unit or bipolar electrode 21 are interleaved between the cathodes 41 of the next adjacent bipolar unit or bipolar electrode 21 forming'a single diaphragm cell. A bipolar unit or bipolar electrode 21 including anode 31 and cathodes 41 and an equalizer 51 is shown in partial cutaway in FIG. 2. The backplate 23 has an anodic surface 25, typically a foil or thin sheet, e.g., on the order of one-sixteenth of an inch or thinner, of an anolyte resistant metal. Most commonly the anolyte resistant metal is a valve metal, Le, a metal which forms a protective oxide film upon exposure to acidic medium under anodic conditions. The valve metals include titanium, hafnium, zirconium, tanatalum, tungsten, columbium, and their alloys. Most commonly titanium is used. Alternatively, the anolyte resistant surface 25 of the backplate 23 may be rubber, e.g., a Neoprene or EPDM rubber sheet.

The anode fingers 31 extend from the anodic surface, of the backplate 25. The anodes 31 are typically coated valve metal sheets, plates, or blades. They may be perforate'd or foraminous or expanded mesh. The coatings are those materials which provide a low chlorine overvoltage, chlorine-resistant surface. Typical of the coating materials are the platinum group metals,'their oxides, and their oxygen-containing compounds.

Spaced from and parallel to the cathodic surface 27 of the backplate 23 is a cathode screen 43. The cathode screen 43 and the cathodic surface 27 of the backplate 23 define a catholyte volume 45. Extending from and in hydraulic communication with the catholyte volume 45 are hollow cathode fingers 41. The cathode fingers may be in the form of perforate metal fingers or metal mesh fingers. The cathodes themselves are most commonly iron or steel, and are covered with a diaphragm or permionic membrane. Most commonly, an asbestos diaphragm is used.

An equalizer 51 extends from the anolyte-resistant surface 25 of the backplate 23 through the backplate 23, including the catholyte resistant surface 27 of the backplate 23, through the catholyte volume 45 and through the cathode backscreen 43. The equalizer may also be considered as passing from the anolyte volume of one cell through the cathode and cathode backscreen 43 of that cell and the catholyte chamber 45 of that cell to the backplate 23 and through the backplate 23 to the anolyte chamber of the next adjacent cell.

According to this invention, the structure of equalizer insures that there is no discharge of anolyte into the'intermediate catholyte chamber between the two anolyte chambers. Additionally the structure of the equalizer prevents seepage of anolyte into the backplate between the anolyte resistant member 25 and the catholyte resistant member 27 of the backplate as well as preventing the seepage of anolyte or catholyte through the backplate 23 around the equalizer 51.

The equalizer itself should have sufficient chemical resistance to both the anolyte and the catholyte to prevent the discharge of anolyte from the equalizer into the catholyte chamber and seepage of catholyte into the anolyte. This may be provided by a single conduit having an interior surface resistant to the anolyte and an exterior surface resistant to the catholyte, and which according to one exemplification may be provided by a single conduit fabricated of a material that is resistant to both the anolyte and the catholyte. Such materials include KYNAR (TM), TEFLON (TM), and similar fluorinated hydrocarbons. Metals resistant to both the izer of this type is shown in FIG. 4 where the equalizer 51 is a single tube fabricated of a fluorocarbon.

According to another exemplification of this invention, the equalizer may include an interior conduit 53 of an anolyte resistant material sheathed 55 with a catholyte resistant material on that portion of the equalizer which passes through the intermediate catholyte chamber.

An electrolyte-tight seal must be provided between the anodic surface 25 of the backplate 23 and the equalizer 51 so as to prevent the seepage of anolyte behind the anolyte resistant surface 25 of the backplate 23. According to one exemplification of this invention shown in FIG. 3, the anolyte resistant member 25 of the backplate 23 is joined to the equalizer 51. This may be accomplished by welding, resistance welding, silver soldering, or the like.

According to another exemplification of this invention, a collar 71 is provided on the equalizer 51. This collar 71 provides a seal between the anolyte resistant member 25 of the backplate 23 and the anolyte. According to the exemplificat'ion of this invention shown in FlG. 4, this seal may be provided by bonding the collar 71 to the anolyte resistant member 25 of the backplate 23. According to another exemplification of this invention, shown in FIG. 5, the collar 71 may be held in compression against the anolyte resistant member 25 of the backplate 23. This may be accomplished by providing a threaded portion 73 on a segment of the equalizer 51 extending from the anolyte resistant member 25 of the backplate 23 into the anolyte chamber and providing a threaded portion 75 on the collar 71 which serves to compress the collar 71 onto the anolyte resistant member 25 of the backplate'23. The collar 71 may additionally be seal welded to the anolyte resistant member 25 of the backplate and to the extension 73 of the equalizer, 51. As shown in FIG. 5, a shoulder 77 on the equalizer 51 compresses the opposite surface, i.e., the catholyte resistant surface 27 of the backplate 23, thereby providing an electrolyte-tight seal at both surfaces 25, 27 of the backplate 23.

An electrolyte-tight seal must be provided between the cathodic member 27 of the backplate 23 and the equalizer 51 so as to prevent the seepage of catholyte liquor into the backplate 23. According to'one exemplification of this invention, this may be accomplished by a lip or collar 81 on the segment of the equalizer 51 exposed to catholyte liquor. The lip or collar 81 is compressed against the catholyte resistant member 27 of the backplate either directly against the catholyte resistant member 27 of the backplate 23, or against a flange or gasket 87 interposed between the catholyteresistant member 27 of the backplate 23 and the collar itself 81.

One catholyte seal is shown in FIGS. 3 and 5 with bolts 85 extending from the catholyte resistant member 27 of the backplate 23, through a gasket 87 and through apertures 89 in the lip or collar 81 on the segment of the equalizer 51 exposed to catholyte liquor. The bolts 85 are fitted with nuts 91 which when tightened provide an electrolyte-tight fit between the catholyte resistant member 27 of the backplate 23, the lip 81 of the equalizer 51, and the gasket 87 therebetween.

According to another exemplification of this invention, the catholyte-tight seal may be provided by a lip 81 on the portion of the equalizer 51 which is exposed to catholyte, the lip 81 being welded to the catholyte resistant member 27 of the backplate 23, as shown in FIG. 4. This weld may be a gold seal weld, a silver weld or even a tantalum weld.

The electrolyte-tight seals between the anolyte resistant member 25 of the backplate 23 and the equalizer 51, and between the catholyte resistant member of the backplate 27 and the equalizer 23 also serve to prevent seepage of electrolyte between the equalizer 51 and the backplate 23 and into the equalizer 51.

Additionally, the equalizer 51 should be constructed so as to permit some movement of the electrolyzer, i.e., thermal expansion, without failure.

As shown in H6. 3 the anolyte side of the equalizer 51 is welded to the anolyte resistant member 25 of the backplate 23 providing an electrolyte-tight seal therebetween. On the catholyte resistant side 27 of the backplate 23 bolts 85 are provided in the backplate 23 corresponding to apertures 89 in the lip 81 around the equalizer 51. A gasket 87 between the lip 81 and the catholyte resistant surface 27 of the backplate 23 is compressed by tightening nuts 91 on the bolts 85 thereby providing an electrolyte-tight seal while permitting some movement and thermal expansion in the cell.

According to another exemplification of this invention shown in FIG. 4, a collar 71 is welded to the anolyte side of the equalizer 51. This collar 71 is also welded to the anolyte resistant member 25of the backplate 23. A collar 81 is also welded to the catholyte side of the equalizer 51. This collar 81 is also welded to the catholyte resistant member 27 of the backplate 23.

A still further exemplification of this invention is shown in FIG. 5 where the segment 73 of the equalizer 51 extending into the anolyte is threaded. The threaded portion 75 of the collar 71 is threaded onto the equalizer 51 providing an electrolyte-tight seal between the anolyte resistant member 25 of the backplate 23 and the equalizer 51. The catholyte resistant member 27 of the backplate 23 has bolts 85 which extend through apertures on a collar 89 of the equalizer 51 corresponding to the bolts 85. The equalizer 51 itself consists of two parts, a catholyte resistant part 101 and an anolyte resistant part 103. The collar 81 on the catholyte resistant member compresses a gasket 87. A shoulder 77 on the anolyte resistant member 103 also compresses the gasket 87 providing a liquid tight seal between the anolyte resistant member 103 of the equalizer 51 and the catholyte resistant member 101 of the equalizer 51 as well as a catholyte tight seal between the catholyte resistant member 101 of the equalizer 51 and the catholyte resistant member 27 of the backplate 23. in this way, some movement of the cell, as well as thermal expansion is permitted without damaging the equalizer or permitting electrolyte seepage or leakage.

In all of the equalizer configurations shown, an electrolyte-tight seal is provided between the equalizer 51 and the cathode screen 43 by the deposited asbestos diaphragm.

It is to be understood that although the invention has been described with specific reference to particular embodiments thereof, it is not to be so limited since changes and alterations therein may be made which are within the full intended scope of this invention as defined by the appended claims.

We claim:

1. In a bipolar electrolyzer comprising a pair of diaphragm cells electrically and mechanically in series and separated by a backplate having a catholyte-resistant surface facing the first of the pair of cells, and an anolyte-resistant surface facing the second of the pair of cells, an electrolyte permeable cathode in said first cell, spaced from and mechanically and electrically connected to the cathodic surface of said backplate and defining a catholyte volume therebetween, brine feed means to each of said cells, and gas recovery and product recovery means from each of said cells, the improvement comprising:

an aperture in said backplate;

bolt means extending from the catholyte-resistant surface of said backplate;

collared, anolyte-resistant conduit means extending through the anolyte-resistant surface of said backplate, the flange thereof bearing upon the catholyte-resistant surface of said backplate;

collared, catholyte-resistant conduit means corresponding to said anolyte-resistant conduit means, and extending from said anolyte-resistant conduit means to said cathode, whereby to provide equalizer means through said catholyte volume between anolyte volumes of said pair of cells independent of said brine feed means, gas recovery means, and product recovery means;

said bolt means passing through the collar of said catholyte-resistant conduit means, and nut means on said bolt means compressively bearing on said collar and said anolyte-resistant flange means whereby to provide an electrolyte tight seal between said anolyte-resistant conduit means and said catholyte-resistant conduit means;

and an electrolyte tight seal between said anolyteresistant conduit means and the anolyte-resistant surface of said backplate.

2. The bipolar electrolyzer of claim 1 wherein the anolyte-resistant conduit is welded to the anolyteresistant surface of the backplate whereby to provide an electrolyte tight seal therebetween.

3. The bipolar electrolyzer of claim 1 comprising:

a threaded portion of said anolyte-resistant conduit extending beyond the anolyte-resistant surface of said backplate;

collar means surrounding said anolyte-resistant conduit and bearing on the anolyte-resistant surface of said backplate; and

nut means on the threaded portion of said anolyteresistant conduit bearing upon said collar means whereby to provide an electrolyte tight seal between said anolyte-resistant conduit and the anolyte-resistant surface of said backplate. 

1. In a bipolar electrolyzer comprising a pair of diaphragm cells electrically and mechanically in series and separated by a backplate having a catholyte-resistant surface facing the first of the pair of cells, and an anolyte-resistant surface facing the second of the pair of cells, an electrolyte permeable cathode in said first cell, spaced from and mechanically and electrically connected to the cathodic surface of said backplate and defining a cathOlyte volume therebetween, brine feed means to each of said cells, and gas recovery and product recovery means from each of said cells, the improvement comprising: an aperture in said backplate; bolt means extending from the catholyte-resistant surface of said backplate; collared, anolyte-resistant conduit means extending through the anolyte-resistant surface of said backplate, the flange thereof bearing upon the catholyte-resistant surface of said backplate; collared, catholyte-resistant conduit means corresponding to said anolyte-resistant conduit means, and extending from said anolyte-resistant conduit means to said cathode, whereby to provide equalizer means through said catholyte volume between anolyte volumes of said pair of cells independent of said brine feed means, gas recovery means, and product recovery means; said bolt means passing through the collar of said catholyteresistant conduit means, and nut means on said bolt means compressively bearing on said collar and said anolyte-resistant flange means whereby to provide an electrolyte tight seal between said anolyte-resistant conduit means and said catholyte-resistant conduit means; and an electrolyte tight seal between said anolyte-resistant conduit means and the anolyte-resistant surface of said backplate.
 2. The bipolar electrolyzer of claim 1 wherein the anolyte-resistant conduit is welded to the anolyte-resistant surface of the backplate whereby to provide an electrolyte tight seal therebetween.
 3. The bipolar electrolyzer of claim 1 comprising: a threaded portion of said anolyte-resistant conduit extending beyond the anolyte-resistant surface of said backplate; collar means surrounding said anolyte-resistant conduit and bearing on the anolyte-resistant surface of said backplate; and nut means on the threaded portion of said anolyte-resistant conduit bearing upon said collar means whereby to provide an electrolyte tight seal between said anolyte-resistant conduit and the anolyte-resistant surface of said backplate. 